1. Field of the Invention
The present invention relates to an image processing technique based on radioscopy images obtained through moving picture imaging such as radioscopy imaging.
2. Description of the Related Art
Due to recent advancements in digital techniques, it has become common to perform digital processing on, for example, images obtained through medical X-ray radioscopy. Two-dimensional X-ray sensors capable of outputting X-ray images as digital data are being developed in place of X-ray imaging using conventional X-ray diagnostic film. Digital image processing, such as tone processing and the like, has become essential in X-ray radioscopy devices that use sensors such as the stated two-dimensional X-ray sensors.
Auto-exposure control (AEC), which detects the amount of X-rays that permeate an object and controls the X-ray amount so that it is neither too high nor too low, is carried out in X-ray radioscopy. In auto-exposure control, a feature amount, such as the average value of an X-ray radioscopy image obtained from X-rays having a pulse-shaped waveform irradiated from an X-ray generation unit 101, is obtained first. Then, the X-ray irradiation conditions (tube voltage, tube current, X-ray pulsewidth of the X-ray generation unit) are controlled based on a comparison between the level of the feature amount and a reference value, in order to achieve a desired exposure.
Image processing, auto-exposure control, and so on performed by an X-ray radioscopy imaging device aim to appropriately display a region of interest corresponding to an anatomical structure in a human body, which is the most important part of the image for diagnostic purposes.
In the image processing, auto-exposure control, and so on performed by an X-ray radioscopy imaging device, the region of interest is extracted from the captured image, and the feature amount used in the image processing, auto-exposure control, and so on is calculated from the extracted region of interest. The region of interest is different depending on the portion to be imaged, the purpose of the imaging, and so on. For example, when performing radioscopy of the stomach using a barium liquid, the stomach wall is taken as the region of interest in order to detect polyps present therein; when capturing moving pictures of the chest area, the lung field region is the region of interest; and in cardiac catheterization, the tip of the catheter and the surrounding region thereof is the region of interest.
Meanwhile, regions outside the exposure field when the exposure field is limited using a collimator, transparent regions where X-rays enter directly into the sensor without passing through the object, and so on are detrimental to the proper calculation of the feature amount and should therefore be left out of the region of interest. In addition, when regions in which the X-ray absorption rate differs greatly from that of the object, such as pieces of metal, are contained within the region of interest, such regions are also detrimental to the proper calculation of the feature amount and should therefore be left out of the region of interest.
Conventionally, setting a threshold for differentiating between a region of interest and other regions, and then performing a thresholding process for extracting the region of interest based on this threshold, an edge extraction process for extracting contour forms of an object based on the form of the gradation distribution of the image, or the like has been used as a method for extracting a region of interest from an image.
For example, Japanese Patent Laid-Open No. 2000-10840 discloses a method for creating a density histogram on the object regions within a radiation image and performing tone correction, dynamic ranging, and so on of the radiation image based on a feature amount of the image calculated from the density histogram. Stable extraction of the feature amount of the object region within the image can be performed by extracting image component information corresponding to the bones, soft tissues, and the like of the object using transparency elimination, the form of the histogram, and so on. Effective image processing is thus possible even in cases where, for example, the maximum pixel density values in the object region of the radiation image are less than a predetermined pixel density value.
Meanwhile, Japanese Patent Laid-Open No. 2005-218581 discloses a method for extracting an exposure field region in order to optimize the image processing parameters. In this method, scores are given regarding how closely a pixel of interest and a pattern of the surrounding pixels thereof resemble the border of the exposure field, and an exposure field candidate region is calculated so as to correspond to a collimator form, such as a circle or a polygon. A form feature amount, such as the degree of circularity, is found for the exposure field candidate region, and the form is identified thereby; the exposure field is then extracted using an exposure field recognition algorithm tailored to the identified shape. As the algorithm tailored to the identified shape, linear detection processing such as a Hough transform is employed for polygons, whereas template matching with circular forms or the like is employed for circles; this increases the accuracy.
Finally, in Japanese Patent Laid-Open No. 2003-250789, a region of interest is extracted to be used for feature amount calculation in order to perform either auto-exposure control, image density conversion, or both as appropriate in radioscopy that generates an image at the comparatively low rate of three to five frames a second. In this method, the image data of a quadrangular exposure field region is first projected (accumulated) in the top, bottom, right, and left directions of the image, after which a one-dimensional array is created for each direction; a secondary differentiation computation is then performed on these arrays. The positions having the maximum values are taken as the external tangents (borderlines) of the exposure field in each direction and cut out, extracting the exposure field; a process for extracting the region of interest is then performed on the obtained result of cutting out the exposure field. The process for extracting the region of interest is performed while switching the extraction algorithm for each region of interest based on information regarding the portion that has been imaged or order information. Algorithms that utilize analysis of image histograms, morphology computations, logical computation of binary images, and so on are disclosed as methods for setting a predetermined region, detecting the stomach wall as the peripheral contour region of a barium mass, and detecting the lung field region when capturing a moving picture of the chest area.
However, generally, algorithms for extracting a region of interest are complicated. In particular, in a device that processes a large amount of data, such as with X-ray radioscopy, it is difficult to extract a region of interest between the X-ray irradiation and the display with high accuracy and at the high frame rate (25 to 30 fps) that is required. The abovementioned conventional techniques are used for extracting a region of interest from an image obtained through still-image capturing or radioscopy at a comparatively low frame rate, and no consideration is given to a technique for extracting an appropriate region of interest from a moving image obtained through high-frame rate radioscopy.